Nervous NATO allies seek reassurance from Trump's defense chief

At their first face-to-face encounter with Jim Mattis, European defense ministers will also seek reassurances from their U.S. counterpart that Trump is committed to the North Atlantic Treaty Organization, which has guaranteed Europe's security for almost 70 years.

"Mattis's performance is going to be really important," said Adam Thomson, a former British ambassador to NATO and director of the European Leadership Network think tank in London. "It is all about how credibly he appears to speak for the entire U.S. administration."

Trump's contradictory remarks on NATO, calling it "obsolete" during the U.S. election campaign but also supporting it, as well as his praise for Russian President Vladimir Putin, have deeply unsettled allies who fear for transatlantic ties.

In his debut trip to Europe as Pentagon chief, Mattis is set to echo longstanding U.S. calls that European allies invest more on defense, something U.S. defense secretaries under Republican and Democratic administrations have done for years.

But experts say that message will have to be calibrated, because while Trump has eased off his criticism of NATO since taking office, he has maintained criticism of some allies for failing to make "full and proper financial contributions."

"Many of them have not been even close," Trump said of European defense spending. "And they have to do that," Trump told U.S. troops in Florida earlier this month.

Moscow's 2014 annexation of Crimea, Ukraine's Black Sea peninsula, and its direct support for rebels in eastern Ukraine have prompted NATO to begin sending alliance troops to Russia's borders to deter the Kremlin from any other moves.

Mattis, a former NATO general who served in the U.S. military for four decades, is likely to feel at home at the alliance headquarters. But he also arrives amid renewed uncertainty in Washington after White House national security adviser Michael Flynn's resigned this week.

Mattis said that resignation over Flynn's pre-inauguration discussions with Russia over economic sanctions had "no impact".

One senior European NATO diplomat said allies are under no illusion that Trump might not still undo careful diplomatic work with a single message on Twitter. But they would still look for his support as they try to manage tensions with Russia.

"We need to bring the family together," the diplomat said. "Things have been made to feel fragile," the diplomat added, referring to Trump's campaign remarks about NATO.